The present invention relates to a small volume prover for proving accuracies of flow meters, and more particularly to a high precision small volume prover capable of proving flow meters without being affected by test fluid pressure and temperature.
Flow meter proving apparatuses are used for conducting periodical and/or optional characteristic tests of flow meters newly fabricated to use and flow meters currently used in lines so that they can measure flow rate at a reliable accuracy without the possible characteristic changes due to the influences of extrinsic factors such as temperature and pressure and also of intrinsic factors such as wearing of movable parts. The characteristic tests are conducted basically by two methods: one is called a calibrating system using a stationary proving apparatus to which a flow meter to be tested is connected, and the other is called a prover system by which a flow meter is proved by being mounted in a fluid system.
Since the above-mentioned prover method can conduct a characteristic test of a flow meter as mounted in a line at any time, it is used mainly for testing conjectural type flow meters, e.g. turbine meters.
Flow meter provers operate on such a common principal that a moving displacer such as a piston or ball travels simultaneously with fluid flow in a cylindrical conduit having a uniform cross-section and displace a known volume of fluid in a predetermined section thereof which is determined as the reference volume. In proving a flow meter by a prover, the corresponding metered volume simultaneously passes through the flow meter and the flow meter's readings, i.e. the whole number of pulses generated by the flow meter is counted to determine a K-factor(meter factor) representing a number of pulses per unit volume displaced. If needs be, a continuous flowrate characteristic curve is plotted on the basis of a K-factor calculated for a plurality of flowrate measurements.
To obtain a high resolution meter factor it is needed to increase a number of pulses generated per base volume over a certain number, for example, 10000 pulses for stationary prover having a large base volume. If the base volume is less than the above-mentioned one, the desired number of meter pulses can not be generated, but a number of clock pulses(time) generated for the period that a displacer such as a piston travels and displaces the base volume of fluid and meter pulses(time) generated directly before and after said duration can be used for determining therefrom a meter factor. Therefore, even in case a smaller number of meter pulses is generated, it is possible to use small type provers (hereinafter called small volume provers) which are portable.
The small volume provers are designed basically such that a movable piston travels through a certain section of a cylindrical measuring conduit of a constant cross-section connected in series with a flow meter to be tested and the flow meter reading is compared to the displaced volume of fluid.
The fluid volume is practically determined from the displacement of the piston. In proving, a plurality of test results are averaged and then a meter factor(K-factor) is determined on the basis of the average value.
For this reason, the piston reciprocates in the cylindrical measuring conduit by the number of flowrate measurements.
Having traveled a given distance in the cylindrical measuring conduit and completed a proving pass, the piston is returned to its initial position by means of a hydraulic or pneumatic actuator driving the piston through a piston rod against the fluid stream. In this case, fluid can pass the cylindrical measuring conduit or a by-path provided in parallel with the measuring conduit.
In case of the fluid passing through the measuring conduit, the piston to be returned by means of the actuator is provided with valve functions for closing the port during the proving pass and opening the port during returning. This method is, hereinafter, called an internal valve method.
In case of fluid passing through a by-path, a by-pass valve is provided to close and open the by-pass, respectively, during a proving pass and returning of the piston. Hereinafter, this method is called an outer valve method.
The Japanese laid open patent publication No. 153063/79 discloses a small volume prover of the internal valve type in which a piston (movable member) includes a poppet valve which is opened to allow only the fluid to pass therethrough while the piston is fixed in a non-measurement state, and is closed to force the piston to travel simultaneously with a fluid stream during a proving pass. However, the poppet valve is frequently operated and, consequently, its seat portion may rapidly wear. In case of small volume provers, fluid leakage through the piston assembly may give a significant influence to their measurement results and, therefore, the reliability of the poppet valve may directly concern the test results.
The Japanese laid open patent publication No. 173418/85 discloses a compact type flow meter prover which is of the external valve type.
FIG. 1 is a view for explaining a conventional small volume prover which includes an inlet pipe 81, an inlet 81a, a housing 82, a by-path 83, a by-pass valve 84, an actuator 85, an introducing portion 86, a displacer(piston) 87, a shaft 88, a main cylinder 89, a downstream portion 90, an outlet pipe 91, a spring 92, journal bearings 93, 94, a hydraulic cylinder 95, a hydraulic piston 96, detecting rod 97, a detecting unit 98, sensors 99, 100, 101, a detecting flag 102, a pilot 103 and a sleeve bearing 104.
The housing 82 is composed of a main cylinder 89 serving as a measuring cylinder, an introducing chamber 86 being larger in diameter than the main cylinder 89 and a downstream portion 90.
The introducing chamber 86 includes a hydraulic cylinder 95 therein and communicates with an inlet pipe 81 connected to the portion adjacent to the cylinder-mounted portion. A by-path of the housing 82 is composed of the inlet pipe 81, a by-pass pipe 83 and an outlet pipe 91 and includes a by-pass valve 84 therein between its inlet 81a and outlet 91a. A hydraulic cylinder 95 and the main cylinder 89 are coaxially arranged and a hydraulic piston 96, a shaft 88 and a displacer 87 are connected in series with each other to form a single member. The shaft 88 is liquid-tightly supported by a journal bearing 93. A spring 92 is provided between the journal bearing 93 and the displacer 87.
The displacer 87 is provided with a detecting rod 97 fixed thereto for detecting a travel of the displacer 87.
As shown in FIG. 1, the displacer 87 currently exists in the introducing chamber 86 of a large diameter and a detection flag 102 coexists with a sensor 99. With a by-pass valve 84 being closed, the fluid introduced through the inlet 81a and the inlet pipe 81 passes through the main cylinder 89 and is discharged through an outlet 91a of an outlet pipe 91. The displacer 87 rests and is ready to start travelling.
When a command to start measurement is given, a hydraulic piston 96 starts moving toward a downstream portion 90 (to the right) to move the displacer 87 with the aid of the spring 92. A base volume of fluid displaced from the cylinder 89 by the displacer 87 is determined as a travel distance of the detection flag between two sensors 100 and 101. The displacer 87 then stops when a pilot 103 extending axially from the displacer rests in a sleeve bearing 104. In this state, the fluid passes the downstream portion 90 of a large diameter and exits from the outlet 91a.
When the by-pass valve 84 is opened by the action of an actuator 85, the fluid passes through the by path 83 and is discharged from the outlet 91a. In this state the displacer 87 is returned to the shown starting position by means of the hydraulic piston 96.
In the above-described small volume prover of the external valve type, the displacer 87 with a seal 87a slidably moves in the main cylinder 89 with no fear of fluid leakage that is encountered in any prover of the internal valve type having a displacer provided with a poppet valve. However, since the main cylinder 89 is a single wall conduit precisely finished to have an uniform diameter, it can be deformed by the influence of temperature and pressure of the fluid. When fluid temperature is high, the main cylinder 89 may have a large difference of temperatures between its outer and inner surfaces that is also dependent upon the fluid temperature. Therefore, it is not easy to correct a change of volume in the main cylinder 89. In addition, a change of the fluid pressure may also change the reference volume of the small volume prover. Such a construction that the by-path line is externally connected to the main cylinder 89 for communication with the introducing chamber 86 and the downstream portion 90 is connected to the portion 90 makes it difficult to reduce the size of the prover.